In the design and manufacture of semiconductor IC (integrated circuit) chip packages and modules, it is imperative to implement mechanisms that can effectively remove heat generated by IC chip devices (such as microprocessors) to ensure continued reliable operation of such devices. Indeed, heat removal is particularly important for computer processor chips that are disposed in SCM (single chip modules) or MCMs (multichip modules), for example, which can generate significant amounts of heat. The ability to efficiently remove heat becomes increasing problematic as chip geometries are scaled down and operating speeds are increased, resulting in increased power density. Although IC chip modules are being continually designed to operate at higher clock frequencies, increased system performance is becoming limited primarily by the ability to effectively remove heat from such IC chip modules.
There are various heat removal techniques that have been developed for cooling semiconductor IC packages/modules and other electronic devices. For example, microchannel cooling apparatus and methods have been proposed for cooling electronic devices such as IC chips, MCMs, diode laser arrays, and other electro-optic devices under conditions of increased heat flux (power/unit area) or high power densities (e.g., ˜800 W/cm2).
FIGS. 1A and 1B are schematic diagrams that illustrate a conventional microchannel cooling apparatus, such as described in U.S. Pat. No. 4,573,067, wherein FIG. 1B illustrates a cross-section view of FIG. 1A along the line 1B. As shown, a semiconductor chip (10) includes circuits that are formed in a front surface region (11) of the semiconductor chip (10). A rear surface of the chip (10) is processed to form a recessed region (12) and a plurality of parallel, microscopic heat conducting fins (14) rising from the recessed region (12), which define a plurality of channels (13). A transparent cover (15) engages the surface of chip (10) and the tops of the fins (14) thereby defining a chamber for the flow of a coolant through the channels (13) between the input and output ports (16) and (17) in the transparent cover (15), wherein heat removal is achieved by thermal contact between the fins (14) and the coolant fluid that flows through the channels (13).
There are a number of disadvantages associated with the cooling apparatus depicted in FIGS. 1A and 1B. For instance, such design results in significant pressure drops and non-uniform flow distribution due to, e.g., the use of one heat exchanger zone (with long channel length) between the input port (16) and the output port (17), and having an input port (16) and output port (17) with a cross sectional area that is smaller than the total microchannel cross sectional area. Furthermore, the process of forming the fins (14) and channels (13) directly in the non-active surface of the IC chip (10) can result in reduced yield for the chips (10), which is not desirable especially for expensive chips such as microprocessors. Indeed, if the microchannel cooler fails or leaks, the chip, which is much more expensive than the cooler in the case of a high performance processor, is lost along with the microchannel cooler.
FIGS. 2A˜2C schematically illustrate another conventional microchannel cooling apparatus, such as described in U.S. Pat. No. 5,998,240. FIG. 2A depicts a silicon chip (20) having a region containing a plurality of microchannels (21) formed therein, which comprise a plurality of close-ended slots or grooves of generally rectangular cross-section. As depicted in FIG. 2B, the chip (20) sits on a ceramic frame (22) that includes three generally rectangular coolant manifolds (23), (24) and (25). The center manifold (24) comprises a coolant input manifold having a coolant inlet port (27) formed at one end, while the two outer manifolds (23) and (25) comprise output manifolds and include coolant outlet ports (26) and (28) at the opposite end from the inlet port (27). The die (20) is oriented with respect to the ceramic substrate (22) such that the microchannels (21) are orthogonal to the manifolds (23), (24) and (25). As depicted in FIG. 2C, the chip (20) and the ceramic substrate (22) are mounted on a ground plane (29) having two coolant output ducts (29a) and a single coolant input duct (29a), wherein the liquid coolant flow direction is shown by the arrows.
There are a number of disadvantages with the conventional microchannel cooling apparatus depicted in FIGS. 2A˜2C. For instance, if the substrate (22) comprising the manifold channels (23, 24, 25) was fabricated using Silicon, the substrate would be weak and likely break during fabrication due to the formation of the multiple channels through the substrate with sharp-edge corners. Moreover, such design results in significant pressure drops and non-uniform flow distribution, which result from (i) the input and output ports (26, 27, 28) having a smaller cross sectional area than the total microchannel cross sectional area, (ii) the manifolds (23, 24, 25) having grooves which are of constant cross section feeding the microchannels (21) and (iii) having the microchannels (21) continue below the inlet manifold groove (24) when two outlet manifold grooves (23, 25) are used.
FIGS. 3A and 3B are schematic diagrams illustrating other conventional microchannel cooling structures, such as described in U.S. Pat. No. 5,218,515. FIG. 3A illustrates a cut-away perspective view of an integrated circuit module (30), which includes an IC chip (31) having solder bump bonding sites (32) along a front (active) surface (31a) of the chip (31). A back (non-active) surface (31b) of the chip (31) is thermally bonded to a microchannel structure (33). A plurality of microchannels (33a) are formed in the microchannel structure (33). A cover manifold (34) is bonded to the microchannel structure (33). Input and output coolant delivery channels (34a) and (34b) are cut or formed in the cover manifold (34) as illustrated.
FIG. 3B illustrates a coolant supply manifold (35) which is used to supply coolant for a multi-chip module (MCM) package comprising an array of the microchannel cooled IC modules (30) mounted face down on a common substrate or circuit board. The coolant supply manifold (35) includes a plurality of coolant supply channels (e.g. (36), (37), (38) and (39)), wherein channels (36) and (38) are higher pressure channels while channels (37) and (39) are lower pressure channels. The manifold (35) is adapted for placement over a printed wiring card so that, e.g., the openings (36a) and (37a) in respective coolant supply channels (36) and (37) mate with the openings (34a) and (34b) in the individual integrated circuit modules (30) on the circuit board. Elastomer seals are used to couple the coolant supply manifold (35) with the integrated circuit modules (30).
There are disadvantages associated with the microchannel cooling apparatus depicted in FIGS. 3A and 3B. For instance, the microchannel coolers (30) are formed with one heat exchanger zone (between the input and output manifold channels (34a, 34b)), which can result in significant pressure drops of fluid flow along the microchannels (33a). Moreover, if a cover manifold (34) was formed with multiple coolant delivery channels (e.g., 34a) for multiple heat exchanger zones to reduce the pressure drop, the cover manifold (34) would be fragile and likely to break during fabrication, thereby reducing manufacturing yield.
Furthermore, the coolant supply manifold (35) design of FIG. 3B can result in large pressure drops and significant non-uniform flow distribution due to the channels, which feed a given column of four microchannel inlets, having a constant cross-sectional area. For instance, as depicted in FIG. 3B, the supply channel (36) feeds coolant fluid to four coolant delivery manifolds (34a) of module (30) (FIG. 3A) via the four openings (36a). Assume V, ΔP, and Q are the velocity, differential pressure and total flow in the last manifold segment, i.e., between the bottom two microchannel inlets, then the segment above will desirably have velocity 2V, and total flow 2 Q. This higher velocity will result in a segment pressure drop equal to ˜2ΔP if the flow is laminar and ˜4ΔP if the flow is turbulent (given the linear dependency between flow and pressure drop when the flow is laminar, but quadratic dependency when the flow is turbulent). Thus, for a manifold channel with constant cross-section feeding four microchannel inlets, we can expect a total manifold channel pressure drop of ˜10 ΔP if the flow is laminar, and ˜30 ΔP if the flow is turbulent. This large pressure drop will induce flow variations within the different microfin sections that can not be compensated with a return line, also with constant cross-section but with reverse orientation relative to the inlet line. Since the manifold channel has to carry significantly larger water flow than a given microchannel, both velocity and cross-section will be higher, therefore, it is easy to expect (or difficult to avoid) having a 10× increase in the Reynolds number between these two sections. For low cooling capability, like less than 50 W/cm2, the water flow requirements are low, and it is possible to have both manifold channels and microchannels with laminar flow regimes, but at high flows this is not possible since the microchannels will require flow regimes with Reynolds number in the 100–200 range, then, the flow regime in the manifold channel will not be laminar.
Another disadvantage associated with the conventional designs discussed above is that no mechanisms are provided for efficient heat removal for IC chips, such as processors, that have “hot spot” regions, i.e., regions of the chip which can have a heat flux (power/unit area) that is significantly greater than the average heat flux of the chip, which could result in temperatures ˜20° C. hotter than the average chip temperature. Indeed, one thermal solution which is designed for efficiently removing heat that is generated due to average chip power density, may not be adequate for removing heat at “hot spot” regions of the chip, which can result in failure of the chip devices in or near the “hot spot” region.